I have a similar problem, but not quite identical. Mine occurs with both the
uniprocessor and SMP kernels. When the system is booted, the network will be
working properly. (In other words, with the SMP kernel, my problem is not as
severe as yours; the network does work to begin with.)
Some time later, the machine will stop responding to the network. Sometimes the
machine will stop responding at all. Other times, the machine will continue
responding to low level requests like ICMP ECHO, while stopping responding to
higher level requests like TCP.
This problem has occurred on two machines, one dual processor, one uniprocessor.
It started with the upgrade to kernel-2.6.12-1.1447_FC4. I was previously
running kernel-2.6.12-1.1398_FC4 and this seems to work fine. I've downgraded
one of the boxes and the lockups seem to have stopped. (The other box needs to
be rebooted by the hosting provider, and I'm still waiting for this to be done.
I will report what happens after it is downgraded.)

I have tried kernel-smp-2.6.12-1.1398_FC4 and symptoms are the same. Logs are
similar. Network tries to boot and lost it in a few seconds. With
kernel-2.6.12-1.1398_FC4 there is no problem, everything works fine.
Obviously, there is a problem with my hardware and smp kernel and I don't know
what it is. Maybe someone could look at logs that I have attached and find
something that I am unable to see.

Mass update to all FC4 bugs:
An update has been released (2.6.13-1.1526_FC4) which rebases to a new upstream
kernel (2.6.13.2). As there were ~3500 changes upstream between this and the
previous kernel, it's possible your bug has been fixed already.
Please retest with this update, and update this bug if necessary.
Thanks.